This invention relates to a method of coloring a water-absorbable plastic by using a coupler and a diazonium double salt which develops a color on the plastic.
Known methods of coloring plastics include the following:
A. A suitable dye or pigment is dispersed or dissolved in a monomer, and the monomer is polymerized to obtain a colored plastic. This method is featured by dispersion of a coloring agent in a plastic.
B. A suitable dye or pigment is mixed and dispersed in pellets or resin, which are then mixed with a suitable plastic to produce a colored plastic. This method is also featured by dispersion of a coloring agent in a plastic.
C. A solution of a suitable dye or pigment is obtained by dissolving the dye or pigment in a suitable solvent, and a plastic is immersed in the solution, whereby the dye or pigment permeates into the plastic and a colored plastic is obtained. This method is also featured by dispersion of a coloring agent in a plastic.
D. A plastic is colored by printing, transfer or coating. This method is featured by coloring a plastic by causing a coloring agent to adhere to the surface of the plastic.
It will be observed that according to any of the above methods `A`, `B` and `C`, a coloring agent is simply dispersed in a plastic without any chemical reaction therewith. Thus, if any of the methods `A`, `B` and `C` is employed to color a water-absorbable plastic, it is impossible to obtain a satisfactorily colored plastic because water entering the plastic causes the migration of a coloring agent within the plastic, and also because water leaving the plastic causes the migration of the coloring agent out of the plastic. All the methods `A`, `B` and `C` have a further disadvantage when used for coloring a water-absorbable plastic, because those methods present difficulty in partial or multiple coloring of a water-absorbable plastic. The method `D` may be used for partial or multiple coloring of a plastic, but cannot be employed satisfactorily for coloring a water-absorbable plastic. If a water-absorbable plastic colored by the method `D` is immersed in water, the plastic swells, but the layer of the coloring agent applied on the surface of the plastic cannot swell so much as the plastic itself does. As a consequence, the coloring agent scales off the plastic surface.